title: Sodium-dependent and calcium-dependent calcium transport by rat brain microsomes.
abstract: Microsomal vesicles prepared from rat brain contain a Na+-Ca2+ exchange transport system capable of accumulating Ca2+ in a time- and temperature-dependent manner. The Ca2+ accumulated by these vesicles was released by the Ca2+ ionophore A23187 but not by EGTA. The Km value for Ca2+ uptake was 23 microM with a maximal velocity of 21 nmol Ca2+/mg per min. Ca2+ uptake was significantly inhibited by La3+, Sr2+, Mn2+ and Ba2+ and to a lesser extent by Mg2+. 45Ca2+ accumulated by Na+-dependent uptake could be released by 40Ca2+, indicating the presence of a Ca2+-Ca2+ exchange activity in the microsomes. Ca2+-Ca2+ exchange was stimulated in Li+- and K+-containing media as compared to choline+ media. Microsomes also catalyzed ATP-dependent Ca2+ uptake (in the absence of Na+ gradient). The Ca2+ sequestered by this mechanism could be released by extravesicular Na+, indicating that both the ATP-dependent and the Na+-dependent Ca2+ uptake systems are present in the same membrane. The microsomal preparation used did not contain measurable amounts of succinate dehydrogenase activity or oligomycin-azide-dinitrophenol sensitive ATP-dependent Ca2+ uptake. Thus, the Ca2+ accumulation observed was not due to contaminating mitochondria. The preparation was enriched for 5'-nucleotidase and (Na+ + K+)-ATPase (plasma membrane markers) as well as antimycin A-resistant NADPH-dependent cytochrome c reductase activity (an endoplasmic reticulum marker).
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002412:Cations; D007700:Kinetics; D008297:Male; D008861:Microsomes / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000494:pharmacology*
chemical_list: D002412:Cations; D000001:Calcimycin; D000255:Adenosine Triphosphate; D012964:Sodium; D002118:Calcium